The invention relates to a resistor, particularly a low-resistance current-measuring resistor, and a coating method for coating such a resistor.
A low-resistance current-measuring resistor is known from EP 0 605 800 A1, which consists of two plate-shaped connection parts consisting of a conductor material (e.g. copper) and a plate-shaped low-resistance resistor element consisting of a resistor material (e.g. copper-manganese-nickel alloy) inserted between the connection parts, the voltage drop across the resistor element according to Ohm's Law forming a measurement for the electrical current flowing through the measuring resistor.
With such a current-measuring resistor, the problem is that, without a coating, the material of the connection parts and the resistor element oxidizes so that such current-measuring resistors then turn brown and look unattractive, the solderability also being impaired through the oxidation.
A known method to solve this problem is to galvanically tin-plate or nickel-plate the composite material strip from which the above-described current-measuring resistors are die-cut so that the resistor element on the finished die-cut current-measuring resistors is provided with a coating at least on the topside and on the underside, said coating preventing the interfering oxidation. However, applying the coating before die cutting has the disadvantage that the die flanks and thus the lateral edges of the resistor element have no coating and therefore continue to be exposed to oxidation.
With stringent customer requirements for surface protection, in the past the entire current-measuring resistor was galvanized, as shown in FIG. 6. From this drawing it can be seen that the current-measuring resistor has two connection parts 1, 2, a resistor element 3 being inserted in the direction of current between the two connection parts 1, 2. The oxidation of the connection parts 1, 2 and the resistor element is prevented in this arrangement by a metallic coating 4, which is applied to the current-measuring resistor by galvanization. The problem here is that the galvanized coating 4 is electrically conductive and can therefore falsify the electrical resistance of the resistor element 3 through an electrical bypass through the coating 4. To avoid such falsification of the resistance value, the free surface of the resistor element is coated circumferentially with an electrically-insulating coating 5, which prevents the interfering by-pass through the metallic coating 4. In this way, the interfering oxidation of the surfaces of the connection parts 1, 2 and the resistor element 3 is fully prevented but applying the coating 5 is extremely laborious and has had to be done manually up till now.
Furthermore, DE 197 80 905 C2, which discloses another resistor type, should be mentioned as prior art. In this arrangement, the resistor element is affixed to an electrically insulating substrate and is covered by a metallic coating on its topside. However, firstly, the metallic coating has a lower resistance value than the resistor element in this arrangement and thus forms a substantial electrical by-pass. Secondly, the lateral edges of the resistor element also remain uncovered in this arrangement and can therefore oxidize.
Furthermore, US 2003/0016118 A1, DE 198 14 388 A1, DE 26 34 232 A1 and DE 260 70 26 A1 should be mentioned as prior art.
The object of the invention is therefore to create an appropriately improved resistor. This object is achieved by a resistor according to the invention and by a corresponding coating method according to the invention.